1. Field of the Invention
This invention relates to methods of applying polymer thick film inks (hereinafter referred to as PTF inks) and devices incorporating PTF inks.
2. Introduction to the Invention
The use of PTF inks is well known in the electronics industry. The inks may have conductive, resistive, or insulating properties, and are typically applied onto a planar substrate in an electronics device, to impart the required electrical properties to that device. The most common use of PTF inks is in membrane switches, in which two flexible, insulating, plastic sheets, one of which is usually polycarbonate and the other of which is usually polyester, are screen printed on their mutually adjacent surfaces with a continuous layer of conductive silver-loaded PTF ink. The printed plastic sheets are separated by an apertured spacer through which electrical contact may be made by applying pressure to the sheets at the position of an aperture. Another known use of PTF inks is in multilayer circuits in which a conductive PTF ink is applied to a substrate, typically a ceramic substrate, in a pattern so as to define an electrical circuit.
PTF inks typically comprise three components (1) a polymeric binder, (2) a solvent (for the binder) and (3) a functional phase (also referred to in the art as a pigment or a filler). In the ink, as supplied, the polymeric binder is dissolved in the solvent and the functional phase is dispersed in that solution.
To apply the ink to a substrate, a screen printing technique is generally used; then the substrate is positioned in an oven to vaporize the solvent from the ink and cure the polymeric binder. The removal of the solvent and curing of the binder occurs in a single heating step. The term "cure" is used herein to include any solidification of the binder, whether or not it is accompanied by chemical reaction of the binder.
The purpose of the components of the ink are as follows. The binder acts as a matrix for the functional phase, binds the ink to the substrate, and gives some abrasion resistance. The solvent dissolves the polymer and controls the viscosity of the ink to provide the flow characteristics necessary for screen printing. The functional phase is generally a particulate filler and is selected to provide the required electrical properties of the ink.
Typical organic binders used in PTF inks include cellulosic resins, epoxies and thermoplastic polymers, for example polyesters, acrylics and vinyl copolymers. Typical solvents include ketones, alcohols, aromatics and chlorinated solvents. The choice of the functional phase depends on the electrical properties of the filler. Particulate silver is typically used for conductive inks, and particulate carbon for resistive inks.